Optical wavelength filters can be used to change the spectral composition of an incoming beam to achieve an output beam of a desired wavelength or within a certain wavelength range. One class of optical wavelength filters is bandpass filters which transmit light within a desired spectral transmission window. The transmission window is centered at a selected center wavelength and has a selected bandwidth.
Bandpass filters have been used in many color display systems to filter one or more light beams of a white light source to produce colors, such as the primary colors red, green, and blue. Each passband filter is designed to transmit one color defined by a selected center wavelength and a spectral range. Colors outside the spectral range are rejected.
Some flat panel displays, such as color liquid crystal displays ("LCDs") in portable computers, use color filter arrays in which each pixel includes three color filters corresponding to three different colors. Many LCDs use dye color filters to form the color filter arrays. Such dye color filters are expensive and have a low transmission efficiency of only about 60%.
Another implementation of the color filter array uses dielectric interference filters. Such filters are well known in the art. A typical dielectric interference filter is formed of alternating dielectric layers with different indices of refraction. The alternating dielectric layers can be configured to produce a transmission window at a desired center wavelength with a desired bandwidth.
The multi-layer interference filters can be easily configured to achieve a desired transmission window such as a notch shape. However, it often requires over ten layers of alternating different dielectric layers to produce desired color filtering and transmission efficiency. Since different colors require different multi-layer structures, a color filter array based on such interference filters are complex in its structure and expensive to manufacture. For example, at least thirty masking processes are needed to form the filters for three colors if each color requires at least ten dielectric layers.